Superconducting magnets are used in a variety of contexts, including nuclear magnetic resonance (NMR) analysis, and magnetic resonance imaging (MRI). To realize superconductivity, a magnet is maintained in a cryogenic environment at a temperature near absolute zero. Typically, the magnet includes one or more electrically conductive coils which are disposed in a cryostat and are cooled by a cryogenic fluid such as liquid helium.
Many superconducting magnets operate in “persistent mode.” In persistent mode, one or more superconducting electrically conductive coils which form the superconducting magnet is initially energized with current from an external power supply to start up its magnetic field. Once the desired magnetic field is obtained, the power supply is disconnected from the magnet and the magnet maintains the current and the magnetic field due to its superconductivity.
To operate in persistent mode, a persistent current switch is typically provided across the electrical leads, which supplies the energizing current to the magnet. During a magnet energization period (e.g., at startup), the persistent current switch is placed into a resistive state such that it allows the superconducting electrically conductive coils to be energized by the current from the power supply. Once the magnet has been energized, the persistent current switch is switched to a superconducting state for normal persistent mode operation of the superconducting magnet.
Persistent mode switches can be switched between a superconducting state and a resistive state by the application of heat. When the persistent current switch is at a cryogenic temperature (e.g., about 4° K), it is in a superconducting state and has near zero resistance. However, when the persistent current switch is heated to a resistive mode temperature, which is typically greater than the superconducting temperature, it is in the resistive state. In the resistive state, the persistent current switch is not “open” like a typical electrical switch, but rather has a resistance typically between a few ohms and tens of ohms.
During charging of the superconducting magnet the persistent current switch is heated to the resistive mode temperature and a voltage is applied across the switch to charge the magnet. This typically dissipates energy in the persistent current switch. Typically, the persistent current switch is located in the very low temperature (cryogenic) environment, and energy dissipated in the switch is typically transferred as a heat load into that environment. This heat may be removed using a low temperature refrigeration system or through boiling cryogens. Refrigeration systems are typically inefficient at removing heat in very low temperature (cryogenic) environments. As a result, this typically requires very large, expensive, resistive persistent current switches to be employed, or costly cryogens are allowed to be boiled off which are replaced after the magnet has been charged. Neither of these things is desirable.
One aspect of the present invention can provide an apparatus including: a cryostat having an enclosure, and a thermal shield disposed within the enclosure, the thermal shield defining an inner region, and further defining a thermal insulation region disposed between the thermal shield and the enclosure; a cold head having a first stage element disposed in the thermal insulation region, and a second stage element disposed in the inner region and being configured to operate at lower temperature than a temperature of the first stage element; a first heat exchange element thermally coupled to the first stage element of the cold head; a second heat exchange element thermally coupled to the second stage element of the cold head; an electrically conductive coil disposed within the enclosure and configured to produce a magnetic field when an electrical current is passed therethrough; a persistent current switch disposed within the enclosure and connected across the electrically conductive coil, wherein the persistent current switch includes a superconducting material which is electrically superconducting at a superconducting temperature and electrically resistive at a resistive mode temperature which is greater than the superconducting temperature; a persistent current switch heater configured to be selectively activated and deactivated so as to heat the persistent current switch to the resistive mode temperature; and a thermally conductive link thermally coupling the persistent current switch to the second heat exchange element, wherein the persistent current switch is thermally coupled via a convective heat dissipation loop to the first heat exchange element, and wherein the thermally conductive link includes a material which has a first thermal conductivity at the superconducting temperature and a second thermal conductivity at a second temperature which is greater than the superconducting temperature, wherein the first thermal conductivity is greater than the second thermal conductivity.
In some embodiments, the apparatus can further include a superconducting convection cooling loop disposed within the enclosure and connected to the second heat exchange element, the superconducting convection cooling loop having a cryogenic fluid disposed therein and being configured to cool the electrically conductive coil to the superconducting temperature.
In some embodiments, the apparatus can further include a controller configured to activate the persistent current switch heater during a magnet energization period wherein the electrically conductive coil is brought to the superconducting temperature and charged to produce the magnetic field with a particular strength, the controller further being configured to deactivate the persistent current switch heater during an operating period following the magnet energization period once the electrically conductive coil is charged to produce the magnetic field with the particular strength.
In some embodiments, during the magnet energization period, more heat can be transferred from the persistent current switch to the first heat exchange element via the convective heat dissipation loop than is transferred from the persistent current switch to the second heat exchange element via the thermally conductive link.
In some embodiments, the superconducting temperature is about 4° K and the first heat exchange element is at a temperature of about 40° K.
Another aspect of the present invention can provide a method of operating a device including an electrically conductive coil, a persistent current switch connected across the electrically conductive coil, a persistent current switch heater, and a first heat exchange element thermally coupled via a convective heat dissipation loop to the persistent current switch, wherein the persistent current switch includes a superconducting material which is superconducting at a superconducting temperature and electrically resistive at a resistive mode temperature which is greater than the superconducting temperature. The method can include, during a magnet energization period: cooling the electrically conductive coil to the superconducting temperature; heating the current switch heater so as to raise a temperature of the persistent current switch to the resistive mode temperature; applying energy to the electrically conductive coil so as to charge the electrically conductive coil to produce a magnetic field with a desired strength; and while applying the energy to the electrically conductive coil, dissipating heat from the persistent current switch to the first heat exchange element via the convective heat dissipation loop.
In some embodiments, the method can further include disposing the first heat exchange element above the persistent current switch, and wherein during the magnet energization period the first heat exchange element is at a first temperature which is greater than the superconducting temperature.
In some embodiments, the first temperature is about 40° K and the superconducting temperature is about 4° K.
In some embodiments, the method can further include during an operating period subsequent to the magnet energization period, dissipating heat from the persistent current switch to a second heat exchange element via a thermally conductive link, wherein the second heat exchange element is at the superconducting temperature during the operating period.
In some embodiments, the thermally conductive link can include a material which has a first thermal conductivity at the superconducting temperature and a second thermal conductivity at a second temperature which is greater than the superconducting temperature, wherein the first thermal conductivity is greater than the second thermal conductivity.
In some embodiments, during the magnet energization period and during the operating period, the first heat exchange element can be at a first temperature which is substantially greater than the superconducting temperature.
In some versions of these embodiments, the superconducting temperature is about 4° K and the first temperature is about 40° K.
Yet another aspect of the present invention can provide an apparatus including: a persistent current switch including a superconducting material which is electrically superconducting at a superconducting temperature and electrically resistive at a resistive mode temperature which is greater than the superconducting temperature; a convective heat dissipation loop thermally coupling the persistent current switch to the first heat exchange element; a second heat exchange element spaced apart from the first heat exchange element; and a thermally conductive link thermally coupling the persistent current switch to the second heat exchange element.
In some embodiments, the convective heat dissipation loop can include a two phase heat pipe.
In some embodiments, the thermally conductive link can include a material which has a first thermal conductivity at the superconducting temperature and a second thermal conductivity at a second temperature which is greater than the superconducting temperature, wherein the first thermal conductivity is greater than the second thermal conductivity.
In some embodiments, the apparatus can further include a cryostat having: an enclosure; and a thermal shield disposed within the enclosure and defining a thermal insulation region disposed between the thermal shield and the enclosure.
In some embodiments, the first heat exchange element can include the thermal shield.
In some embodiments, the thermal insulation region can include a vacuum.
In some embodiments, a persistent current switch heater configured to be selectively activated and deactivated can be provided, wherein when the persistent current switch heater is activated it heats the persistent current switch to the resistive mode temperature; a first heat exchange element.
In some embodiments, the apparatus can further include a superconducting convection cooling loop disposed within the enclosure and connected to the second heat exchange element, the superconducting convection cooling loop having a cryogenic fluid disposed therein and being configured to cool the persistent current switch to the superconducting temperature.